Wireless transceivers are used in a wide variety of wireless systems. A wireless transceiver (transmitter and receiver) may typically include a wireless receiver for receiving and demodulating signals, and a transmitter for modulating signals for transmission. Wireless systems typically include a multiple-stage power amplifier to amplify and transmit signals. The power amplifier stages may be coupled together via transformers. The transformer may be used for inter-stage matching between the stages of the power amplifier. The inductance of the transformer also may be used to resonate out any capacitance in the power amplifier. The transformer can also perform impedance transformation to provide the power amplifier an optimal load. It can also be used to supply DC power to the amplifier and bias the transistors of the amplifier.
Wireless systems are increasingly designed to target high-speed and high-power transmission applications. To achieve the high-power performance of the wireless transceiver, the power amplifier is designed with large transistors, which in effect produces a large capacitance at the input and output of the power amplifier. As operating frequencies also increase, the capacitance of the power amplifier becomes a dominant factor that determines the resonant inductance. Given that the resonant frequency can be expressed as f=1/(2π√{square root over (LC)}), where C is the capacitance of the power amplifier, and L is the inductance of the transformer, the inductance available to resonate out the power amplifier capacitance decreases. The capacitance of relatively large transistors for high-power applications make the corresponding resonant inductance unfeasible at millimeter-wave frequencies under transformer design approaches. Furthermore, transformer design approaches implemented by full loops suffer from significant routing parasitic inductance which can be comparable to the inductance of the coupled loop.